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PROLOG

       This  manual  page  is part of the POSIX Programmer's Manual.  The Linux implementation of this interface
       may differ (consult the corresponding Linux manual page for details of Linux behavior), or the  interface
       may not be implemented on Linux.

NAME

       ln — link files

SYNOPSIS

       ln [−fs] [−L|−P] source_file target_file

       ln [−fs] [−L|−P] source_file... target_dir

DESCRIPTION

In the first synopsis form, the ln utility shall create a new directory entry (link) at the destination
path specified by the target_file operand. If the −s option is specified, a symbolic link shall be
created for the file specified by the source_file operand. This first synopsis form shall be assumed when
the final operand does not name an existing directory; if more than two operands are specified and the
final is not an existing directory, an error shall result.

In the second synopsis form, the ln utility shall create a new directory entry (link), or if the −s
option is specified a symbolic link, for each file specified by a source_file operand, at a destination
path in the existing directory named by target_dir.

If the last operand specifies an existing file of a type not specified by the System Interfaces volume of
POSIX.1‐2008, the behavior is implementation-defined.

The corresponding destination path for each source_file shall be the concatenation of the target
directory pathname, a <slash> character if the target directory pathname did not end in a <slash>, and
the last pathname component of the source_file. The second synopsis form shall be assumed when the final
operand names an existing directory.

For each source_file:

1. If the destination path exists and was created by a previous step, it is unspecified whether ln shall
write a diagnostic message to standard error, do nothing more with the current source_file, and go on
to any remaining source_files; or will continue processing the current source_file. If the
destination path exists:

a. If the −f option is not specified, ln shall write a diagnostic message to standard error, do
nothing more with the current source_file, and go on to any remaining source_files.

b. If destination names the same directory entry as the current source_file ln shall write a
diagnostic message to standard error, do nothing more with the current source_file, and go on to
any remaining source_files.

c. Actions shall be performed equivalent to the unlink() function defined in the System Interfaces
volume of POSIX.1‐2008, called using destination as the path argument. If this fails for any
reason, ln shall write a diagnostic message to standard error, do nothing more with the current
source_file, and go on to any remaining source_files.

2. If the −s option is specified, actions shall be performed equivalent to the symlink() function with
source_file as the path1 argument and the destination path as the path2 argument. The ln utility
shall do nothing more with source_file and shall go on to any remaining files.

3. If source_file is a symbolic link:

a. If the −P option is in effect, actions shall be performed equivalent to the linkat() function
with source_file as the path1 argument, the destination path as the path2 argument, AT_FDCWD as
the fd1 and fd2 arguments, and zero as the flag argument.

b. If the −L option is in effect, actions shall be performed equivalent to the linkat() function
with source_file as the path1 argument, the destination path as the path2 argument, AT_FDCWD as
the fd1 and fd2 arguments, and AT_SYMLINK_FOLLOW as the flag argument.

The ln utility shall do nothing more with source_file and shall go on to any remaining files.

4. Actions shall be performed equivalent to the link() function defined in the System Interfaces volume
of POSIX.1‐2008 using source_file as the path1 argument, and the destination path as the path2
argument.

OPTIONS

       The ln utility shall conform to the Base Definitions volume of POSIX.1‐2008, Section 12.2, Utility Syntax
       Guidelines.

       The following options shall be supported:

       −f        Force existing destination pathnames to be removed to allow the link.

       −L        For  each  source_file operand that names a file of type symbolic link, create a (hard) link to
                 the file referenced by the symbolic link.

       −P        For each source_file operand that names a file of type symbolic link, create a (hard)  link  to
                 the symbolic link itself.

       −s        Create  symbolic  links  instead  of  hard  links. If the −s option is specified, the −L and −P
                 options shall be silently ignored.

       Specifying more than one of the mutually-exclusive options −L and −P shall not be  considered  an  error.
       The  last option specified shall determine the behavior of the utility (unless the −s option causes it to
       be ignored).

       If the −s option is not specified and neither a −L nor a −P option is specified,  it  is  implementation-
       defined which of the −L and −P options will be used as the default.

OPERANDS

       The following operands shall be supported:

       source_file
                 A  pathname  of a file to be linked. If the −s option is specified, no restrictions on the type
                 of file or on its existence shall be made. If  the  −s  option  is  not  specified,  whether  a
                 directory can be linked is implementation-defined.

       target_file
                 The pathname of the new directory entry to be created.

       target_dir
                 A pathname of an existing directory in which the new directory entries are created.

STDIN

       Not used.

INPUT FILES

       None.

ENVIRONMENT VARIABLES

       The following environment variables shall affect the execution of ln:

       LANG      Provide a default value for the internationalization variables that are unset or null. (See the
                 Base Definitions volume of POSIX.1‐2008, Section 8.2, Internationalization  Variables  for  the
                 precedence   of   internationalization  variables  used  to  determine  the  values  of  locale
                 categories.)

       LC_ALL    If set to a non-empty string value, override the values of all the  other  internationalization
                 variables.

       LC_CTYPE  Determine  the  locale  for the interpretation of sequences of bytes of text data as characters
                 (for example, single-byte as opposed to multi-byte characters in arguments).

       LC_MESSAGES
                 Determine the locale that should be used to  affect  the  format  and  contents  of  diagnostic
                 messages written to standard error.

       NLSPATH   Determine the location of message catalogs for the processing of LC_MESSAGES.

ASYNCHRONOUS EVENTS

       Default.

STDOUT

       Not used.

STDERR

       The standard error shall be used only for diagnostic messages.

OUTPUT FILES

       None.

EXTENDED DESCRIPTION

       None.

EXIT STATUS

       The following exit values shall be returned:

        0    All the specified files were linked successfully.

       >0    An error occurred.

CONSEQUENCES OF ERRORS

       Default.

       The following sections are informative.

APPLICATION USAGE

       None.

EXAMPLES

       None.

RATIONALE

The CONSEQUENCES OF ERRORS section does not require ln −f a b to remove b if a subsequent link operation
would fail.

Some historic versions of ln (including the one specified by the SVID) unlink the destination file, if it
exists, by default. If the mode does not permit writing, these versions prompt for confirmation before
attempting the unlink. In these versions the −f option causes ln not to attempt to prompt for
confirmation.

This allows ln to succeed in creating links when the target file already exists, even if the file itself
is not writable (although the directory must be). Early proposals specified this functionality.

This volume of POSIX.1‐2008 does not allow the ln utility to unlink existing destination paths by default
for the following reasons:

* The ln utility has historically been used to provide locking for shell applications, a usage that is
incompatible with ln unlinking the destination path by default. There was no corresponding technical
advantage to adding this functionality.

* This functionality gave ln the ability to destroy the link structure of files, which changes the
historical behavior of ln.

* This functionality is easily replicated with a combination of rm and ln.

* It is not historical practice in many systems; BSD and BSD-derived systems do not support this
behavior. Unfortunately, whichever behavior is selected can cause scripts written expecting the other
behavior to fail.

* It is preferable that ln perform in the same manner as the link() function, which does not permit the
target to exist already.

This volume of POSIX.1‐2008 retains the −f option to provide support for shell scripts depending on the
SVID semantics. It seems likely that shell scripts would not be written to handle prompting by ln and
would therefore have specified the −f option.

The −f option is an undocumented feature of many historical versions of the ln utility, allowing linking
to directories. These versions require modification.

Early proposals of this volume of POSIX.1‐2008 also required a −i option, which behaved like the −i
options in cp and mv, prompting for confirmation before unlinking existing files. This was not historical
practice for the ln utility and has been omitted.

The −L and −P options allow for implementing both common behaviors of the ln utility. Earlier versions of
this standard did not specify these options and required the behavior now described for the −L option.
Many systems by default or as an alternative provided a non-conforming ln utility with the behavior now
described for the −P option. Since applications could not rely on ln following links in practice, the −L
and −P options were added to specify the desired behavior for the application.

The −L and −P options are ignored when −s is specified in order to allow an alias to be created to alter
the default behavior when creating hard links (for example, alias ln='ln −L'). They serve no purpose
when −s is specified, since source_file is then just a string to be used as the contents of the created
symbolic link and need not exist as a file.

The specification ensures that ln a a with or without the −f option will not unlink the file a. Earlier
versions of this standard were unclear in this case.

FUTURE DIRECTIONS

       None.

COPYRIGHT

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition,
       Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open  Group  Base
       Specifications  Issue 7, Copyright (C) 2013 by the Institute of Electrical and Electronics Engineers, Inc
       and The Open Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In  the  event
       of  any  discrepancy between this version and the original IEEE and The Open Group Standard, the original
       IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at
       http://www.unix.org/online.html .

       Any  typographical  or formatting errors that appear in this page are most likely to have been introduced
       during  the  conversion  of  the  source  files  to  man  page  format.  To  report  such   errors,   see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .